PThe purpose of the essay is to illustrate the structure and role the structure of the respiratory and cardiac systems, including the circulation of blood.
All living cells need a persistent source of oxygen to carry on their metabolism. The diaphragm separates the abdominal cavity from the chest cavity and the lower respiratory. The upper respiratory tract includes nose, pharynx, trachea, bronchi, bronchioles, alveolar ducts and alveoli, while the lower respiratory tract known as the thoracic cage, consists of the larynx and the trachea which then splits into two bronchi. The thoracic cage is protected by bones and includes lungs, sternum and spine. The nose has hairs covered by mucus that sifters the air and captures all the particles while prevents them from going down. The lungs are cone shaped and cover most the chest cavity. The bronchi are split into the bronchioles leading the alveolar.
Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the external gaseous environment and the blood which occurs in the alveolar region of the lungs.
Fig1: The respiratory system, McKesson health solutions, 2005,
The alveoli have a large surface area that allows the best possible gaseous exchange. When alveoli collapse, the surface area decreases leading to less gaseous exchange occurs. The alveoli and capillaries walls have a layer of epithelial cells which are flat for the gases to break through only two thin cells. It allows short diffusion pathway. If diffusion was longer than the gaseous exchange to take a long process and some gases may be lost to other tissues leading the health problems.
The presence of water from the alveoli cells keeps it moist which allows the oxygen to dissolve. It allows the gases to be dissolved and transported in the solution making diffusion easy. Dry environment makes it difficult to exchange through cells even if they are only one layer cell thick and reduces the tension. Abrahams stated that Surfactant is a soapy solution that lowers the surface tension of the alveoli lining which prevent collapsing of the alveoli. When bacteria manage to pass through the mucus, the phagocyte cells in the alveoli kills them. Alveoli are tiny hollow sacs which cluster together surrounding them are capillaries commonly called capillary beds. The concentration gradient should be maintained steeply. Gases will move from high level of concentration to a low one.
The plasma also called the fluid part of the blood is clear, straw-coloured, watery fluid, like the fluid found in an ordinary blister. It is a yellowish fluid that is about 90% water, and the rest is made up of various substances in solution. The components of plasma include water, mineral salts, plasma proteins, gases, waste products, hormones, enzymes, foods stuff, antibodies and antitoxins.
Fresh water bathes all the body cells and renews the water within the cells. It maintains the blood fluid allows it to circulate the body. The salts in the plasma are necessary for the building of proto-plasma, and they act as buffer substances neutralising acids or alkalis in the body and maintaining the correct ph of the blood.
There are three proteins which are present in plasma; these are three essential proteins found in plasma are albumin, clotting (coagulation) factors and immunoglobulins. The protein gives the blood the sticky consistency called viscosity which is necessary to prevent too much fluid passing through the capillary walls into the tissues. Its function is to transport copper, iron, and immune system. The viscosity of the blood also assists in the maintenance of the blood pressure. The plasma proteins have vital transport functions as they bind many hormones and minerals. Albumin oncotic pressure and binds hormones and drugs, while fibrinogen and prothrombin are produced in the liver and have a mechanism to clot the blood. Heparin prevents the blood clotting in the vessels.
Foodstuffs are in the form of glucose, amino acids, fatty acids and glycerol which are absorbed from the alimentary tract into the blood. Antibodies and antitoxins are complex proteins substances which provide protection against infection and neutralise the poisonous bacterial toxins. Enzymes are chemical substances produced by the body, which produces chemical changes in other substances without themselves entering the reaction.
There are three types of blood cells: erythrocytes (red blood cells), leucocytes (white blood cells) and thrombocytes (platelets). The red cells are minute disc-shaped bodies, concave on either side. They are very numerous around 5 000 000 per cubic millimetre of blood. They have no nucleus but contain a unique protein called haemoglobin. Because of no nucleus, they cannot repair themselves and have a life span of only 100-120 days. The shape and flexibility of the red cells allow them to deform easily and pass through capillaries.
Fig 2: red blood cell side view and top view, Cummings, 2001,
Haemoglobin is a yellow pigment which contains iron, and this iron is important to normal health. It has a great attraction for oxygen and is responsible for the caring of it and plays a role in acid-base buffering.
The function of the red blood cells is to carry oxygen to the tissues from the lungs and to carry away some carbon dioxide. Oxygen in air is inhaled and travels through the lungs to the alveoli. It is diffused into the surrounding capillaries. The oxygen then binds with the haemoglobin of the red blood cells to create oxyhemoglobin. The oxygenated blood enters the pulmonary circulation system leading from lungs to the heart.
In the lungs, dark red blood cells deoxygenated haemoglobin will combine with oxygen to form bright red oxygenated haemoglobin. In the capillaries of the tissues, the oxyhaemoglobin will give up oxygen molecules and revert to deoxygenated haemoglobin. It is summarised below:
In lungs
Haemoglobin + oxygen oxyhaemoglobin
In the tissues
oxyhaemoglobin Haemoglobin + oxygen
fig 3:
Blood enters the left side of the heart from the pulmonary vein into the atrium and down the bicuspid valve into the left ventricle.
Fig 4: The blood flow of the circulatory system, cancerhelp UK,
The blood is pumped out of the heart through the aortic valve into the aorta. Through arteries, veins and capillaries the blood goes around the body where it is needed. The oxygen diffuses from capillaries and into cells while carbon dioxide diffuses into the blood. The deoxygenated blood makes its way back towards through the venules and veins. The blood enters the right side of the heart by the vena cava then into the right atrium leading to passing the tricuspid valve and the right ventricle.
The red blood cells pick carbon dioxide to return to the lungs after carried the oxygen to the cells. Carbon dioxide is transported in three ways including dissolved into plasma, haemoglobin and reactions with the water in plasma which forms carbonic acid that converts into bicarbonate and hydrogen ions.
The artery is elastic and has thick walls which flex the blood pressure. The outer layer of the artery is the tunica adventia which forms a thin fibrous covering. While the tunica media has elastic properties to allow the stretching outwards with the pressure of the heart. The inner part of the artery is the tunica intima. It carries oxygenated blood away from the lungs besides the pulmonary artery. It is up to 3cm in diameter fig 5: structure of artery
Veins are large blood vessels which are thinner and stiffer walls with one-way valves which prevent the blood from going back the wrong way. Like the arteries walls, they have three layers: an inner part endothelium, a middle layer, the tunica media, and an external fibrous covering the tunica adventia as shown in fig:6 carries deoxygenated blood back to the lungs except for the pulmonary vein which carries blood towards the heart.
Fig 6: structure of Vein,
Capillaries are smallest and most numerous blood vessels which are very delicate. They are an important part of circulation for they are the site of exchange with the tissues cells where internal respiration takes place.
fig 7: structure of capillary,
The heart is a hollow, muscular, cone-shaped organ. It lies between the lungs in an area called the mediastinum, behind the body of the sternum with two-thirds of its bulk on the left side. The heart measures about 12 cm from base to apex, about 9 cm in width and is about 6 cm thick. It is divided from base to top by a muscular partition called the septum. The two sides of the heart have no communication with each other in health. Each side is separated into upper and lower chamber.
fig 8: the structure of heart, 1997, Mader,
The top chamber is the atrium which is smaller and is a receiving chamber of blood flows through veins. The ventricle, the lower chamber of which the blood goes into the arteries. Each atrium connects with the ventricles below on the same side of the heart through an opening, called the atrio-ventricular valve.
It is the events that occur during one beat of the heart. The cycle starts towards the end of the diastole when the whole heart is relaxed. The atrioventricular valves (right tricuspid and left mitral) are open due to the atrial pressure with slight increase ventricular pressure. The cardiac cycle starts when the heart is relaxed when the blood in the veins is at a higher pressure than the atria hence the blood goes from the veins to the atria and then the ventricles. The systole is where the heart contractions begin, which causes the increase in pressure that pushes blood to the ventricles and a delay of around 0.1 seconds occurs. After the delay ventricles start to contract and the blood pressure increases in the ventricle than the atria, the tricuspid and mitral valve closes. The contraction continues leading to the pressure in ventricles to build up resulting in the opening of the arterial valve allowing blood flow into the arteries. Contractions will proceed in the ventricles leading to reaching the high level of blood pressure in the pulmonary artery and the aorta. The blood pressure goes down slowly as the blood starts moving away from the heart and to the lungs. When the heart relaxes and blood pressure carries on falling that leads to the occurrence of diastole
The sinoatrial node as part of the heart intrinsic conduction system controls the rhythm of the heart. Once the cells are alive, the heart will beat input from the nervous system. Automaticity is the automatic nature of the heartbeat which is due to the electrical activity of the sinoatrial node. The sinoatrial node generates electrical impulse which spreads through the heart via s-nodal tissue pathway that coordinates the events of the cardiac cycle. The electrical impulse regulates the heart beat that causes the individual reading of an ECG.
Fig 9:
Fig 9 shows the electrical impulse spreads from the sinoatrial node to the walls of the atria causing them to contract. The impulse reaches the atrioventricular node making a 0.1-second delay. Bundle branches carry signals from the atrioventricular node to the heart apex. The signal spreads through the ventricle walls resulting in contraction. The final stage of the ECG cycle is to prepare the next heartbeat.
Cardiac output is the volume of the blood pumped every minute (Ward at el, 2013). The heart rate and stroke volume: heart rate x stroke volume determine the output. An echocardiograph measures how much blood is pumped out of the ventricle in each stroke. The standard rate is 5l per minute which can rise to about 30 litres per minute. By this formula, if stroke volume is 80 and heartbeat of 70, then the cardiac output will be 5600ml (5,6l). Cardiac output is necessary to indicate problems and determines the effectiveness of the heart.
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